Nitrate contamination of drinking water is increasingly common in many countries, intake of nitrate induces the formation of nitrite in human body, which has harmful effects on human health. Therefore, reliable and efficient removal of nitrate in drinking water receives more and more attention.
Nitrate in drinking water may be removed by physical, chemical and biochemical methods. According to the types of electron donor, biochemical denitrification may be divided into two categories: autotrophic and heterotrophic denitrification. Common electron donors for autotrophic denitrification are elemental sulfur and hydrogen gas. Bacterial yield of autotrophic denitrification is low, so is its processing efficiency. Acidity and sulfate produced in sulfur-supported autotrophic denitrification may reduce the pH and the quality of the processed water. The use of calcium carbonate for adjusting the pH of such treated water will subsequently increase the water hardness. Secondary contamination derived from hydrogenotrophic denitrification is also relatively small, but the low solubility of hydrogen gas causes operational difficulties in transport/utilization of hydrogen gas and control of pH during hydrogenotrophic denitrification. So, it is not easy to operate hydrogenotrophic denitrification on large scale.
Compared to autotrophic denitrification, heterotrophic denitrification has a higher processing capability per unit volume. Fixed bed and fluidized bed bioreactors are commonly used for heterotrophic denitrification treatment of nitrate-contaminated drinking water. A fixed-bed bioreactor needs to overcome the problem of clogging in the long run. For a fluidized bed bioreactor, the operational cost is high to maintain a high processing ability. A conventional activated sludge-based approach, widely used for nitrate removal in wastewater treatment, is not readily suitable for nitrate removal from drinking water, mainly because it is poor at precise control of the addition of organic carbon source during denitrification and the formation of water soluble organic matter, suspended solids and residual nitrite in the produced water, and hard to meet the high requirements of drinking water.
Among various activated sludge-based approaches, operation of activated sludge in sequencing batch mode under automatic control is competitive in terms of controlling the quality of produced water and flexibility in operational scale. Activated sludge treatment of nitrate contaminated drinking water on large scale is possible, if taking proper measures to improve the settling property of conventional activated sludge, adopting an organic carbon-dominated and hydrogen gas-supplemented electron donor supply strategy, and operating in sequencing batch mode, in order to avoid excessive addition of organic carbon source and to reliably control the formation of water soluble organics, suspended solids and nitrite in the produced water.